A POF includes an inner core having a refractive index distribution and an outer core provided on periphery of the inner core. A ratio Ra/Rb of an outer diameter of the inner core to the outer core satisfies 0.67.ltoreq.(Ra/Rb).ltoreq.0.87 such that an outgoing beam from an end of the POF may have a parallel area of at least 200 .mu.m. Thus a space between the end for exiting the exit light and a light receiving device can be kept at least 200 mm without members or devices for collimating.